Estrogen Receptor Protein Interaction with Phosphatidylinositol 3-Kinase Leads to Activation of Phosphorylated Akt and Extracellular Signal-Regulated Kinase 1/2 in the Same Population of Cortical Neurons: A Unified Mechanism of Estrogen Action

Abstract

17␤-Estradiol (E 2 )-induced neuroprotection is dependent on mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI3K) signaling cascades. We sought to determine whether E 2 neuroprotective mechanisms are mediated by a unified signaling cascade activated by estrogen receptor (ER)-PI3K interaction within the same population of neurons or whether E 2 activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt are independent signaling events in different neuronal populations. Immunoprecipitation of E 2 -treated cortical neurons was conducted to determine a protein-protein interaction between ER and the PI3K regulatory subunit p85. Subsequently, cortical neurons were treated with E 2 alone or in presence of MAPK inhibitors or PI3K inhibitors. Results of these analyses indicated a protein-protein interaction between ER and p85 that was time-dependent and consistent with the temporal profile for generation of Akt (pAkt) and ERK1/2 phosphorylation (pERK1/2). E 2 -induced phosphorylation of Akt, was first apparent at 10 min and maximal at 30 min. Simultaneously, E 2 -induced pERK1/2 was first apparent at 5-10 min and maximal at 30 min. Inhibition of PI3K completely blocked E 2 activation of pAkt at 10 and 30 min and blocked E 2 activation of ERK1/2 at 10 min, which revealed a PI3K-independent activation of ERK at 30 min. Double immunocytochemical labeling for pERK1/2 and pAkt demonstrated that E 2 induced both signaling pathways in the same neurons. These results indicate a unified signaling mechanism for rapid E 2 action that leads to the coordinated activation of both pERK1/2 and pAkt in the same population of neurons. Implications of these results for understanding estrogen mechanism of action in neurons and therapeutic development are considered.

Modulation of αCaMKII signaling by rapid ERα action

“…Numerous studies indicate that E2's rapid effects in the brain are initiated outside of the nucleus (Kuroki, 2000;Mannella, 2006). Our ERα localization data in NLT neurons corresponds with other groups in that "classical" ERs are positioned not only in the nucleus, but in the cytoplasm and outgrowths as well (Clarke, 2000;Hart, 2007) where they are available to interact with cytoplasmic signaling cascades.…”

EstradiolIn VivoRegulation of Brain Mitochondrial Proteome

“…Estrogen's neuroprotective effects are multifaceted, encompassing chemical, biochemical, and genomic mechanisms and falling into three mechanistic categories: antioxidant, defense, and viability (Nilsen and Brinton, 2004;Morrison et al, 2006). Our findings demonstrate that a protein/protein interaction between estrogen receptor and the regulatory subunit p85 activates the phosphoinositide kinase-3 (PI3K) signaling cascade, simultaneously activating both the Akt and extracellular signal-regulated kinase (ERK) pathways which ultimately converge on mitochondria (Mannella and Brinton, 2006). Activation of this complex signaling cascade results in a proactive defense state conferring significant protection against Ca 2ϩ dysregulation induced by neurodegenerative insults, leading to greater survival of E 2 responsive neurons (Nilsen and Brinton, 2003;Brewer et al, 2006;Chen et al, 2006).…”

“…Estrogen receptors have been detected in mitochondria (Chen et al, 2004;Yang et al, 2004;Stirone et al, 2005;Yager and Chen, 2007) as well as in the nucleus of neurons (McEwen et al, 2001). In addition to classical ERs, membrane sites of estrogen action, which activate the PI3K/ PKC/Src/MEK/ERK signaling pathway, activating CREB (cAMP response element-binding protein), have been identified as required for E 2 -inducible neuroprotection (Zhao et al, 2004;Wu et al, 2005;Mannella and Brinton, 2006). Although the mechanisms whereby ERs coordinate the complex signaling pathway between three signaling compartments, membrane, mitochondria, and nucleus, remains to be determined, it is striking that ERs are perfectly positioned to coordinate events at the membrane with events in the mitochondria and nucleus.…”

Neurotrophic and neuroprotective actions of estrogen: Basic mechanisms and clinical implications

“…Estrogen has also been shown to rapidly activate the serine/threonine kinase, Akt in hippocampal and cortical neurons [65,69,70,80,204,205,240,314]. The activation of Akt was recently shown to occur in the same cortical neurons as activation of extracellular signalregulated kinases (ERKs) [314], which is intriguing as both Akt and ERK have been implicated to play an important role in regulation of translation in neurons [315 and 316, for review].…”